Characterisation of crystallographic texture of hot rolled low carbon microalloyed steels by Electron BackScatter Diffraction (EBSD)

摘要

Electron BackScatter Diffraction (EBSD) is based on acquisition of diffraction patterns of massive samples during SEM analysis [1, 2]. This technique allows analysis of samples that permit the correct definition of the micro-texture of the samples [1, 3, 4]. This paper will focus on the determination through EBSD of crystallographic textures on low carbon microalloyed steels produced by hot rolling in a thin slab casting with direct rolling plant, in order to understand the possibility to perform the in-line heat treatment; two coiling temperatures are considered here, the first is fixed at 680°C and the second one at 580°C. In these steels the textures are strongly influenced by planned thermo-mechanical history during hot deformation: processing schedules at a coiling temperature within the completely ferritic region show the typical crystallographic orientations induced by the geometry of rolling tools, featured also by some orientation gradients along the thickness. The variation of texture from the centre to the surface of the strip is largely due to the deformative state, characterized by a strong frictional effect between rolls and the same strip, and also influenced by rolls geometry and thermal profile. The steels are plastically deformed in the austenitic state and this causes a wide dispersion of the texture components resulting at the end of the process, but the phase transformation from austenite to ferrite is not able to delete the effects of the hightemperature deformation on textures, because there are well defined relations between orientations in the prior austenitic phase and the ones developed in the ferritic phase. In the same way, the temperatures at the end of rolling and at the coiling stage can play an important role too; actually higher temperatures can promote grain growth phenomena, associated with the selection of those texture components characterized by a greater growth rate [5]. Other phenomena, i.e. precipitation of carbo-nitrides, can occur influencing nucleation and growth rate of crystalline grains.